Cemented carbide tools are used in, amongst others, the tool industry and usually are composed of tungsten carbide grains and cobalt as a matrix. In order to achieve an improvement of their surface properties, these tools are coated, depending on the application purpose, with a hard material layer such as, for example, titanium nitride or chromium nitride, by means of vacuum coating methods. The hard material layers may be present, depending on the application purpose of the tool, as a single layer or as a multi-layer, and they include at least one of the chemical elements Al, Ti, Cr, Si, which are in the form of oxides, nitrides, carbides or mixed compounds, e.g. carbonitrides. These hard material layers are also referred to as ceramic layers.
A decoating of the hard material layer, namely of a ceramic layer, becomes necessary if the tool is to be used again after use and re-grinding or if a defective coating is to be removed from the tool. The difficulty about decoating is caused, on the one hand, by the various applied materials that are used in a hard material layer and in the need to know whether multiple layers or a single layer are present, and, on the other hand, by the chemical instability of the cemented carbide as such.
Tools made of high speed steel are coated with the same hard material layers as cemented carbide tools. However, they are less expensive in manufacturing and due to their chemical resistance they are much easier to decoat than the cemented carbide tools.
Decoating processes are divided into groups according to various hard material layers, wherein a first group comprises Ti and Al based layers on cemented carbide tools and high speed steel tools, e. g. TiN, TiCN, TiAlN, AlTiN, TiAlN/SiN, that are present as mono-block layer, gradient-layer or multi-layer. In this case a decoating method is customary which is based on the wet-chemical removal of hard material layers using complex compositions of hydrogen peroxide solutions, and in which the cemented carbide tool is typically protected by applying a protection voltage. The decoating time when starting from a 2 μm thickness mono-block hard material layer is between 4 to 24 h and thus is very long. Similarly, the consumption of chemicals which need to be constantly renewed in the case of these very long decoating times is very high. This method fails in the case of complex layer systems such as, for example, AlTiCrN. A decoating is no longer possible.
In the case of high speed steel tools, also a wet-chemical removal of hard material layers using complex compositions of hydrogen peroxide solutions is performed, which is done without applying a protection voltage on the tool but instead at increased temperature. The decoating time when starting from a 2 μm thickness mono-block hard material layer is between 1 to 4 h.
A second group comprises Cr based layers on cemented carbide tools and high speed steel tools, e. g. CrN, AlCrN. In this case a decoating method is customary for both types of tools, which is based on the wet-chemical application of a mixture of permanganate solution and lye. Here, the consumption of chemicals is low and the decoating times of a hard material layer with a thickness of 2 μm is around 1 hour, which is relatively short.
A third group comprises CrTi based layers on cemented carbide tools and high speed steel tools, e. g. CrTiN, AlTiCrN. For these hard material layer systems with highly complex structure no chemical decoating procedures on cemented carbide tools are known. Such coated tools had to be decoated by means of mechanical methods, and the effort therefor is very high.
The decoating of high speed steel tools is based on an electrochemical method which relies on an alkaline peroxide solution with a complex composition as electrolyte. The chemicals are consumed rapidly during decoating, and accordingly the effort is very high. Moreover, this method fails in the case of some variants of AlTiCrN hard material layers.
Further decoating processes available on the market also work in the wet-chemical domain and yield good results in respect of the vulnerability of the cemented carbide tools concerning cemented carbide layer systems of the 1st and 2nd group. However, the decoating time was also unacceptably high. In the field of the decoating of the first and second group of the high speed steel tools the known processes have similar concepts as the above mentioned method.
If the known decoating processes are to be used for ceramic hard material layer systems of the third group, as far as they are applicable at all, very slow decoating times of substantially more than 24 h have to be accepted for cemented carbide tools.
The table below shows an overview of hard material layers that are known and used in industrial practice sorted by groups and by adhesion promoting layers.
Adhesion#Layer typeLayer structurelayerGroup1TiNTiNTiN12TiCNTiN + TiCNTiN13TiAlNTiAlN—14TiN + TiAlNTiN15AlTiNAlTiN—16TiN + AlTiNTiN17TiAlN/SiNTiN + TiAlN/SiNTiN18TiN + AlTiN/SiNTiN19TiN + AlTiN + TiAlN/SiNTiN110TiN + TiAlN/SiN + TiN/SiNTiN111TiAlN/SiN/TiN + TiAlN/SiN + AlCrONTiN?AlCrON12TiAlCrN/SiNTiN/CrN + TiAlN/SiN + CrN o. TiN?AlTiCrN/SiN + TiN/SiN13TiN/CrN + TiAlCrN/SiNCrN o. TiN314TiN/CrN + AlTiCrN/SiNCrN o. TiN315CrNCrNCrN216AlCrNAlCrN—217CrN + AlCrNCrN218AlCrN/TiAlNCrN + AlCrN + TiAlNCrN219AlCrN/SiNCrN + AlCrN/SiNCrN220CrN + AlCrN + AlCrN/SiNCrN221CrTiNCrTiNCrN or TiN322AlTiCrNAlTiCrN—323TiN/CrN + AlTiCrNCrN or TiN324TiN/CrN + AlCrN + AlTiCrNCrN or TiN325TiN/CrN + AlCrN + AlCrTiNCrN or TiN3
A method for decoating of cemented carbide tools is known from WO 99/54528 A1 which allows breaking off of a hard material layer from the cemented carbide tool. Thereby, a tungsten oxide layer is electrolytically formed on the cemented carbide tool which has to be subsequently removed with a mechanical post-treatment. This method is very fast, as it promises decoating times for the first and second group of less then 30 min. A disadvantage here is the need of mechanical post-treatment of the tungsten oxide layer being formed.
From WO 2003/085174 A2 there is known a method which removes surface regions from components by means of pulsed current. As an exemplary component is indicated a turbine blade made of nickel cobalt superalloy. The layer to be removed is metallic and has, in particular, the composition MCrAlY, wherein M is an element of the group of iron, cobalt or nickel. The method known from WO 2003/085174 A2 in the form disclosed therein is not suitable for decoating of ceramic layers of workpieces, namely of steel and hard metal substrates having a ceramic hard material layer on part of their surface.